The present invention relates to apparatus and methods for impingement cooling of turbine components and particularly relates to apparatus and methods for steam cooling turbine shrouds and retrieval of post-impingement cooling steam.
Current methods for cooling turbine shrouds employ an air impingement plate which has a multiplicity of holes for flowing air through the impingement plate at relatively high velocity due to a pressure difference across the plate. The high velocity flow through the holes, strikes and impinges on the component to be cooled. After striking and cooling the component, the post-impingement air finds its way to the lowest pressure sink leakage. However, as this spent cooling air travels to the leakage sink, the accumulating spent air crosses the paths of other high velocity jets of air which are directed to impinge on the component to be cooled. This cross flow of the spent air interacts with the high velocity incoming impingement cooling air to significantly degrade the effectiveness of the impingement cooling air as it crosses from the impingement plate to the component to be cooled.
To applicants' knowledge, an impingement cooling system using steam as the cooling medium for turbine shrouds has not heretofore been developed. Existing air impingement cooling apparatus and methods cannot be used for cooling with steam because post-impingement steam would leak into the turbine flow path. This would be unacceptable from a turbine-efficiency standpoint. A steam impingement cooling system for the turbine shroud must therefore be a closed system with only relatively insignificant leakage of steam.
In accordance with the present invention, there is provided apparatus and methods for impingement cooling of turbine components, particularly, a turbine shroud, using steam as the cooling medium. Specifically, an impingement plate having a plurality of flow passages or apertures through the plate is situated within a homing. The impingement plate defines with opposite walls of the housings a pair of chambers on opposite sides of the plate. Edges of the impingement plate are disposed in slots formed in the side walls and an end wall of the housing, the plate being inserted through a through-slot in the opposite end wall. Once the slot is inserted into the housing to define the chambers, the plate end extending through the through-slot opening in the end wall is welded shut to preclude leakage of steam from the housing as well as to maintain the impingement plate within the housing. The plate is not otherwise welded or braised to the shroud, but is seated in the slots about the housing.
As a consequence of this construction, the chambers on opposite sides of the impingement plate define cooling medium receiving and exhaust chambers. Thus, as the steam enters the system through an inlet pipe welded to a top wall of the housing, the steam supplied the first chamber finds the only available path for further flow is through the holes in the impingement plate. Accordingly, the steam passes through the holes at a substantial increase in velocity and is thereby directed for flow into the second chamber at high velocity and impingement against the shroud surface comprising the opposite or second wall of the housing. By impinging against the shroud surface, the surface is cooled.
In accordance with the present invention, low pressure pockets are provided in the walls of the homing axially along each circumferential wall of the housing. Radially outwardly, there is provided a manifold along the opposite walls of the housing, a plurality of passages communicating between the manifold and an exhaust passage carried by the wall of the housing.
Preferably, the containment wall on the supply side of the housing is pyramidal in shape with the highest area in the center where steam inlet and exhaust pipes are secured. This geometry provides for mixing of the steam in the plenum (corresponding to the first chamber) prior to impingement and ensures uniform distribution of steam to all of the impingement holes through the impingement plate. The passages between the manifold and the common exhaust passage may be cast in the first wall of the housing. The passages from the manifold along opposite side walls of the housing are wide and narrow and follow the length of the manifold. With the pyramidal shape of the housing wall, the passage narrows towards the exhaust passage.
In another form of the present invention, the impingement plate per se includes a plurality of longitudinally extending compartments. A first set of the plurality of compartments comprises cooling medium supply compartments having apertures or openings passing through upper and lower surfaces of the compartment for flowing cooling steam from the first chamber through the apertures into the compartments and through the lower apertures into the second chamber for impingement cooling of the shroud surface. The second set of compartments has a plurality of apertures or openings in communication with the second chamber for receiving the post-impingement cooling steam and directing that spent steam to an exhaust manifold located at one end of the compartment. Preferably, the compartments extend longitudinally of the plate and alternate one with the other throughout their lengths whereby the cooling impingement steam directed against the shroud surface by the aperture of a compartment of a first set thereof is returned after cooling to one or more laterally adjacent compartments and eventually to the exhaust passage. In one form of this invention, a plurality of sleeves may be disposed on the return apertures, such that the sleeves open directly adjacent the shroud surface being cooled.
In a preferred embodiment according to the present invention, there is provided an impingement steam cooling apparatus for turbines comprising a turbine shroud having first and second walls spaced from one another and an impingement plate spaced between the walls to define on opposite sides of the impingement plate first and second chambers substantially sealed from one another, the impingement plate having a plurality of flow passages therethrough providing for communication of cooling steam between the chambers through the passages, a supply passage in communication with the first chamber for supplying cooling steam into the first chamber for flow through the passages and affording impingement cooling of the second wall and an exhaust passage in communication with the second chamber for exhausting post-impingement cooling steam from the second chamber.
In a further preferred embodiment according to the present invention, there is provided a method of cooling a turbine shroud by steam impingement on the shroud comprising the steps of flowing cooling steam into a first chamber within a substantially sealed housing, flowing cooling steam from the first chamber through a plurality of apertures disposed in an impingement plate dividing the housing into the first chamber and a second chamber on the side of the impingement plate opposite the first chamber and directing the steam flowing through the apertures for passage across the second chamber for impingement against the shroud to cool the shroud, and flowing post-impingement cooling steam in the second chamber to an exhaust passage.
Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for steam impingement cooling of turbine shrouds and retrieval of the post-impingement cooling steam.